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Evidences of Chemical Reactions

All chemical bonds posses potential energy.  In a chemical reaction this energy is changed when old bonds are broken and new bonds are formed. Chemical changes are different from physical changes.  When a physical change occurs there is no breaking and forming of bonds.  There are certain things that will help us identify if a chemical reaction has taken place.  We call these evidences of chemical reactions.  

  1. Light is given off: Potential energy in valance electrons is released in the form of light.
  2. Gas is given off: Indicates an increase in kinetic energy of the atom (compound) as it is changed to a gas.
  3. Temperature change: Exothermic is an increase in the speed of the molecules (heat). Exothermic is the taking in of energy.
  4. Color change: the increase or decrease of potential energy stored in new bonds.
  5. Precipitate formed: increase or decrease of potential energy in new bonds formed.
  6. Electrical potential: The release of chemical bond energies to cause the movement of  valence electrons.

All chemical reactions, whether simple or complex, involve changes in substances. One or more starting substances, the reactants, are changed into one or more new substances, the products.

Reactants à Products

In a chemical reaction the ways in which atoms are joined together are changed. Bonds are broken and new bonds are formed as reactants are converted into products. The atoms are not created or destroyed. They are just rearranged.

Chemical reactions can be described in different ways. For example, we could say: "Iron reacts with oxygen to produce iron (III) oxide (rust)." Alternatively, we could identify the reactants and product in this reaction by writing a word equation.

Iron + oxygen à iron (III) oxide

In a word equation, the reactants are written on the left, and the products are written on the right. They are connected by an arrow ( à ) that is read as "yields" or "reacts to produce." Word equations communicate the reaction but can get cumbersome. To be more efficient chemists use chemical formulas for writing equations. For example the rusting of iron would be represented as follows:

Fe + O2 à Fe2O3

Equations that show just the formulas of the reactants and products are called skeleton equations. A skeleton equation is a chemical equation that does not indicate the relative amounts of the reactants and products. The physical state of a substance in a reaction can be indicated in the equation by using the following symbols: (s) for solid, (l) for liquid, (g) for gas, and (aq) for a solution in water. These usually follow the substance in the equation and can also be written as subscripts. To write a skeleton equation, you must write the correct formulas of the reactants and products with the reactants on the left side of the yield sign and the products on the right.

Just writing the skeleton equation of reaction does not always correctly communicate what is happening in the reaction. To represent chemical reactions correctly, equations must be balanced so that they show the amount of reactants and products in the reaction. In every balanced equation each side of the equation has the same number of atoms of each element. This is necessary to be consistent with the law of conservation of mass. Remember, in a chemical reaction, atoms are not created nor destroyed; they are simply rearranged. You can find some examples and help on the tutor link.

Many chemical equations can be balanced by trial and error, but a few guidelines will make it easier and quicker.

Rules for balancing equations:

  1. Write the skeleton equations, either word or formula, to represent the facts.
  2. Determine the correct formulas for all the reactants and products.
  3. Write the formulas for the reactants on the left and formulas for the products on the right with a arrow in between. If two or more chemicals are involved, separate their formulas with "+" signs.
  4. Balance the elements one at a time by using coefficients. (small whole numbers that appear in front of a formula in an equation.) when no coefficient is written, it is assumed to be 1. It is best to begin with an element other than hydrogen or oxygen. These two elements often occur more than twice in an equation. You must not attempt to balance an equation by changing the subscripts in the chemical formulas of a substance.
  5. Finally, make sure that all the coefficients are in the lowest possible ratio.
  6. We can only know with certainty what the products of a chemical reaction are by carrying out that reaction in the laboratory. The reactants must be allowed to react and the products of this chemical reaction must be identified. Carrying out each reaction in the lab is the ideal, but it is both time consuming and costly. It is possible, however, to predict the products of some chemical reactions. To achieve this, you must be able to recognize various types of reactions. Five general types of reactions are: composition, decomposition, single replacement, double replacement, and combustion.

     

    Composition

    Composition is when two or more substances react to form a single substance. The reactants of most common composition reactions are either two elements or two compounds. The product of a composition reaction must be a compound. Two nonmetals can often combine in more than one way. Thus for composition reactions involving nonmetals you will usually need to be told what the product is.

    R + S à RS

    Example: Ca + S ® CaS

    Decomposition

    In a decomposition reaction a single compound is broken down into two or more simpler products. These products can be any combination of elements and compounds. Most decomposition reactions require energy in the form of heat, light or electricity in order for them to occur. When a simple binary compound breaks down the products will be the constituent elements. When a metallic carbonate is heated it will break down to form it metallic oxide and carbon dioxide. Many metallic hydroxides when heated decompose into metallic oxides and water. When a metallic chlorate is heated it will decompose into its metallic chloride and oxygen. Some acids will decompose into nonmetallic oxides and water when heated. And some oxides when heated will also decompose.

    RS ® R + S

    Example: 2HgO ® 2Hg + O2

    Single replacement

    In a single replacement reaction atoms of an element replace the atoms of a second element in a compound. These reactions are also called displacement reactions. Whether one metal will replace another metal from a compound can be determined by their relative reactivities of the two metals. A reactive metal will replace any metal that is less active than itself. Metals will also replace hydrogen in water, and hydrogen in acids. A nonmetal can also replace another nonmetal from a compound. This replacement is usually limited to the halogens. Remember reactivities from bonded for life packet.

    T + RS ® TS + R

    Example: Cu + AgCl ® CuCl2 + Ag

    Double replacement

    Double replacement reactions involve an exchange of positive ions between two compounds. These reactions generally take place between two ionic compounds in aqueous solution.

    R+S- + T+U- ® T+S- + R+U-

    Example: NaCl + AgNO3 ® NaNO3 + AgCl

    Combustion

    In a combustion reaction oxygen reacts with another substance, often producing energy in the form of heat and light. Combustion reactions commonly involve hydrocarbons which are compounds of hydrogen and carbon. The complete combustion of a hydrocarbon produces the compounds carbon dioxide and water.

    CxHy + O2 ® xCO2 + y/2H2O

    Example: CH4 + O2 ® CO2 + 2H2O

    Energy

     Energy is the ability to do work, more simply it is stored work or work in action.

    Kinetic energy is the energy an object possesses due to its motion. The faster the velocity of the object, the more kinetic energy it has. Atoms and molecules are in constant motion, so they have kinetic energy. This is true even in ice. Although fixed somewhat in position in the ice crystal the molecules vibrate back and forth. However, this vibrational energy is small compared with the energy of chemical bonds.

    Potential energy is the energy an object has due to its position relative to other objects. It is the amount of energy stored in something because of its relative position. To understand how the position of atoms is connected to energy, consider the reaction between the elements hydrogen and oxygen to form water. The process of forming water releases a large amount of heat.

     There are several types of ways to express energy. Some examples are:

    1. Chemical: the energy stored in chemical bonds that can be released with the change in electron orbitals.
    2. Mechanical: the ability to do mechanical things, such as raising or lowering, expanding or contracting.
    3. Nuclear: when matter is converted to energy, usually in the form of radiation.
    4. Electrical: is expressed in the flow of electrons from one place to another.
    5. Electromagnetic: when energy is in the form of the electromagnetic spectrum, such as visible light.
    6. Heat: when the energy is contained by the rapid speed of molecules or atoms in motion.
    7.  

      Chemical energy is the energy that matter possesses because of its chemical makeup. If we were to look at a water molecule we would see that the electrons in each atom are moving, so they have energy. There is also energy associated with the force of attraction that holds hydrogen and oxygen atoms together. This is known as the chemical bond.

      A chemical reaction is just the rearranging of atoms. In order for these atoms to be rearranged bonds must be broken and new bonds must be formed. All chemical reactions involve some sort of energy change as the bonds change between atoms. Because the position of the atoms is changing the potential energy of the atoms is changed. Sometimes energy is given off (potential energy is released) or energy is taken in (potential energy of the newly formed bonds is increased).

      Energy changes are evident in chemical reactions when potential energy in chemical bonds is changed in the following manners:

      1. Light is given off: potential energy from valence electrons is released in the form of light.
      2. Gas (bubbles) is given off: indicates an increase in the kinetic energy of the atoms (compounds) as it is changed to a gas. Remember that the state (gas, liquid, solid) of a molecule depends on how fast it is moving. The increased kinetic energy of the molecules can also be changed to mechanical energy by blowing up balloons, powering pistons in car engines, or shooting rockets into space.
      3. Electrical potentials (voltages): is the release of chemical bond energies to cause the movement of valence electrons.
      4. Temperature changes: there are two types: exothermic, where chemical bond potential energy is released by increasing the speed of molecules (you feel heat given off); and endothermic where energy is taken in from the surrounding to increase the potential energy of the new bonds being formed (you feel it as being cold as heat energy is withdrawn from you).
      5. Precipitate formed: it can be from increasing or decreasing the potential energy stored in the new bonds being formed. In a way, this is the same as endothermic and exothermic, only the temperature changes may be too small for you to feel.
      6. Color changes: it can be from increasing or decreasing the potential energy stored in the new bonds being formed. In a way, this is the same as endothermic and exothermic, only the temperature changes may be too small for you to feel.

For information on stoichiometry go to this link

http://www.shodor.org/UNChem/basic/stoic/index.html

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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